1. Field of the Invention
The present invention relates to a focus detection unit and an optical measuring instrument having the focus detection unit. More specifically, it relates to a focus detection unit for forming image of first focusing pattern and second focusing pattern onto an image surface of an optical system and projecting the image onto a workpiece to be observed to adjust relative position of the first and the second focusing pattern for focusing and an optical measuring instrument having the focus detection unit.
2. Description of Related Art
In a field of optical measuring instrument such as microscope, linear image deviation alignment method using a split prism and image sandwiching method where a single sight line is sandwiched between a double sight line for focusing are known as focusing method.
In the former method, since the size of the split prism becomes large enough to cover the entire workpiece because of a complicated structure, vertical line of the target image shifts in right and left at the center in out-of-focus state, thus making focusing difficult.
In the latter method, since two optical systems are required, optical system becomes complicated and adjustment thereof can be troublesome.
In view of the above, the applicant has proposed a focus detection unit for solving the above problems (U.S. Pat. No. 5,963,366).
The focus detection unit is for focusing by forming images of double sight lines and an image of a single sight line on an image surface of an optical system of a microscope, projecting the images onto a workpiece, and adjusting the images such that the image of the single sight line is sandwiched by the images of the double sight lines, including: a first light source; a double slit mask provided on an optical axis of the first light source for forming the double slight lines; a single slit mask provided on an optical axis of the second light source for forming the single sight line; a second light source; a single slit mask provided on an optical axis of the second light source for forming the single sight line; a triangular-pole-shaped prism having a first side surface and a second side surface together constituting an edge of a predetermined angle and a plane opposing the edge for reflecting the double sight lines irradiating the first side surface and the single sight line irradiating the second side surface toward an area before the edge; and a projection lens provided before the edge of the prism for projecting the images of the double sight lines and the image of the single sight line onto the workpiece.
However, following disadvantage occurs in the above-described focus detection unit since the size of the pattern image remains constant relative to the predesignated observation magnification.
(1) When the double sight line image and the single sight line image are projected onto the measurement surface to be sandwiched thereon and the width of the line images coincide with the size of kerfs of the machine processing surface, it can be difficult to recognize the images because the width of the line images are fixed. For instance, in a workpiece having irregularities as shown in FIGS. (A) and (B), it can be difficult to adjust the gap of the slit images influenced by level difference when the irregularities coincide with the slit images.
(2) The magnification of the objective has to be changed for focusing to a microscopic part. Accordingly, when a measuring microscope has only one objective, the lens itself has to be exchanged, thus deteriorating workability.
(3) Since the illumination light source is disposed on two locations, in spite of small number of optical members, two lamps are required, thus increasing production cost.
An object of the present invention is to provide a focus detection unit capable of being used for measuring various workpiece while reducing production cost and an optical measuring instrument having the focus detection unit.
For achieving the object, the focus detection unit and the optical measuring instrument according to the present invention includes following arrangement.
The focus detection unit according to the present invention is for focusing by forming images of first focusing pattern and second focusing pattern onto image surface of an optical system, projecting the images onto a workpiece, and adjusting relative position of the first focusing pattern and the second focusing pattern, including: a focusing pattern forming member having a plurality of pair of the first focusing pattern and the second focusing pattern; a switching member for switching the focusing pattern forming member so that the images of either one of the focusing patterns and the second focusing pattern corresponding thereto on the focusing pattern forming member are projected onto the workpiece; and a projector for projecting the images of the focusing patterns switched by the switching member onto the workpiece.
According to the present invention, since the focusing pattern forming member has the plurality of the first and the second focusing pattern, which is switched by the switching member, the focus detection unit can be suitably used for various workpiece.
For instance, when a machined surface of the workpiece having irregularities is to be focused, easy focusing is possible by switching the focusing pattern that does not coincide with kerfs of the machined surface.
Further, when a microscopic portion is to be focused using a measuring microscope having only one objective, focusing without exchanging lens is possible by switching to a focusing pattern having different size.
In the focus detection unit according to the present invention, the first focusing pattern may preferably include first circular band pattern and the second focusing pattern may preferably include second focusing pattern concentric with the first circular band pattern and having different distance from a center of the first circular band pattern.
According to the above arrangement, since the focusing pattern has the circular band pattern, visibility of the focusing pattern can be improved even on a workpiece having directional property or roughness, thus enabling stable focusing. Especially, by watching change in width of the gap between the circular bands, accurate focusing irrespective of direction of the workpiece can be conducted.
In the focus detection unit according to the present invention, the first focusing pattern may preferably include first semi-circular band pattern and the second focusing pattern may preferably include second semi-circular band pattern line-symmetrical with the first semi-circular pattern.
The above xe2x80x9csemi-circular bandxe2x80x9d shape refers to any shape where the first and the second focusing pattern forms a closed loop in in-focus state, such as semi-circular shape and reverse U-shape.
In the focus detection unit according to the present invention, the first focusing pattern may preferably include first lined triangle pattern having a plurality of lined triangle and the second focusing pattern may preferably include at least two second lined triangle patterns sandwiching the first lined triangle pattern.
Though any profile can be used for the triangle constituting the first and the second focusing pattern, a regular triangle and an isosceles triangle may preferably be used.
In the focus detection unit according to the present invention, the first focusing pattern may preferably include a large rhomboid pattern having a plurality of rhomboid section thereinside and the second focusing pattern may preferably include a plurality of small rhomboid pattern to be disposed in the rhomboid sections of the large rhomboid pattern.
The same effect as the aforesaid circular band pattern can be obtained by the above respective patterns.
In the focus detection unit according to the present invention, the first focusing pattern and the second focusing pattern may preferably be configured so that illuminated portions thereof coincide with each other in in-focus state, and the illuminated portions of both of the focusing patterns may preferably be illuminated by colors to be complementary with each other.
According to the above arrangement, the first focusing pattern and the second focusing pattern are illuminated by colors to be complementary, so that the color vanishes at portion where the patterns are superposed. Accordingly, focusing can be conducted by decoloring the patterns and the workpiece can be more easily observed since the color of the pattern image is lost at the in-focus position. Further, no extinguishing or exchanging work of the pattern is necessary, operability can be further improved.
In the focus detection unit according to the present invention, the projector may preferably include: a light source disposed on an optical axis; a first pair of mirror for reflecting luminous flux orthogonally irradiated by the light source toward the optical axis to irradiate onto either one of the first focusing patterns and the second focusing pattern corresponding thereto provided on the focusing pattern forming member; a second pair of mirror for reflecting the luminous flux transmitted through the focusing pattern forming member toward the optical axis; a prism disposed on the optical axis between the second pair of mirror for entering the luminous flux reflected by respective mirrors into a pair of slope; and a projection lens disposed in front of the prism.
According to the above arrangement, since the projector is constituted of the light source, two pairs of mirror, the prism and the projection lens, in other words, the projector has only a single light source, the production cost can be reduced as compared with the conventional arrangement of having two illumination light sources.
In the focus detection unit according to the present invention, the projector may preferably include: an optical fiber disposed on an optical axis; a first prism disposed on an end of the optical fiber; a first pair of mirror for reflecting luminous flux orthogonally reflected by the first prism toward the optical axis to irradiate onto either one of the first focusing patterns and the second focusing pattern corresponding thereto provided on the focusing pattern forming member; a second pair of mirror for reflecting the luminous flux transmitted through the focusing pattern forming member toward the optical axis; a second prism disposed on the optical axis between the second pair of mirror for entering the luminous flux reflected by respective mirrors into a pair of slope; and a projection lens disposed in front of the second prism.
According to the above arrangement, since the optical fiber and the prism are used instead of the light source of the aforesaid arrangement, thermal influence against the optical system can be reduced while obtaining the same effect as the aforesaid arrangement.
The optical measuring instrument according to another aspect of the present invention has the focus detection unit of the aforesaid respective arrangements and an objective for forming the image projected by the projector onto the workpiece.
According to the present invention, since the optical measuring instrument has the focus detection unit capable of switching the first and the second focusing patterns by the switching member, dimension of various workpiece can be accurately measured.